I put Tchaikovsky - Swan Lake Op.20 - Act II Concl.mp3 in one of my videos. After uploading it to YouTube I was told AdShare MG for a Third Party On behalf of: Naxos claimed a copyright for this. Maybe this information is useful for you.
I had a similar thing happen with "Also sprach Zarathustra" (Not obtained from this website) on of my videos. The supposed copyright holder never responded to YouTube's request and so they let it go. I had also mentioned that the song has been in the public domain for about a century!